Group-III nitride (often referred to as III-nitride, or III-N) compounds, such as gallium nitride (GaN) and its related alloys, have been under intense research in recent years due to their promising applications in electronic and optoelectronic devices. Particular examples of potential optoelectronic devices include blue light emitting diodes and laser diodes, and ultra-violet (UV) photo-detectors. The large bandgap and high electron saturation velocity of the III-nitride compounds also make them excellent candidates for applications in high temperature and high-speed power electronics.
Due to the high equilibrium pressure of nitrogen at typical growth temperatures, it is extremely difficult to obtain GaN bulk crystals. Owing to the lack of feasible bulk growth methods, GaN is commonly deposited epitaxially on substrates such as SiC and sapphire (Al2O3). However, a current problem with the manufacture of GaN thin films is that there is no readily available suitable substrate material whose lattice constant and thermal expansion coefficient closely matching that of GaN. Among the possible substrates for GaN, silicon substrates were explored, although the lattices of silicon substrates do not match that of GaN. Silicon substrates are attractive for GaN growth given their low cost, large diameter, high crystal and surface quality, controllable electrical conductivity, and high thermal conductivity. The use of silicon substrates promises easy integration of GaN based optoelectronic devices with silicon-based electronic devices.
Additionally, due to the lacking of appropriate substrates for growing GaN films thereon, the sizes of the GaN films are limited. Large GaN films will result in great stresses between the GaN films and the underlying substrates, and hence causing the bowing of the substrates. This may cause several adverse effects. First, a great number of defects (dislocations) will be generated in the supposedly crystalline GaN films. Second, the thicknesses of the resulting GaN films will be less uniform, causing wavelength shifts of the light emitted by the optical devices formed on the GaN films. Third, cracks may be generated in large stressed GaN films. New methods for forming GaN films free from the above-discussed problems are thus needed.